I just found this info on a wrangler site so I am not sure if it pertains to us. I jsut thought it was a good read on the np231 transfer case. You might look into the offset issue as well before you begin the swap.
"The NP231J is not compatible with earlier Jeep models because Chrysler changed the offset of the front differential from the passenger side (right-hand drop) to the driver side (left-hand drop) for the Wrangler YJ. This renders the NP231J useless in earlier Jeep models without an axle swap.
NP231J Specifications
Low-gear ratio: 2:72.1
High-gear ratio: 1:1
Reduction type: 3-pinion planetary
Shift pattern: 2H-4H-N-4L
Torque capacity: 1,885 ft-lbs
U-joint size: 1310
Weight: 65 lbs
Length: 16 inches
Fluid capacity: 2.5 pints
Input shaft: 23-spline
Front output shaft: 11⁄8-inch, 27-spline
Numerically Defining the NP231J
You didn’t think they just pulled the “NP231” designation out of a hat did you? As with all transfer cases produced by New Venture Gear, the NP231J nomenclature can be broken down and defined. The “NP” stands for “New Process,” which indicates that the unit was manufactured by New Process Gear. The first number indicates that the NP231J is a two-speed case with high and low ranges. The second number designates the strength of the case. The NP231J rates a “3” in strength, as opposed to the “4” rating given to the NVG241OR transfer case (the largest New Venture Gear transfer case is designated a “7”). The final number reveals the type of transfer case, with the “1” specifying the NP231J as a part-time 4WD case. The “J,” rather obviously, stands for Jeep. Other models of the NP231 are designated “D” for Dodge and “C” for Chevrolet."